The present invention relates to the field of micro electromechanical devices such as ink jet printers. The present invention will be described herein with reference to Micro Electro Mechanical Inkjet technology. However, it will be appreciated that the invention does have broader applications to other micro electromechanical devices, e.g. micro electromechanical pumps or micro electromechanical movers.
Micro electromechanical devices are becoming increasingly popular and normally involve the creation of devices on the xcexcm (micron) scale utilising semiconductor fabrication techniques. For a recent review on micro-mechanical devices, reference is made to the article xe2x80x9cThe Broad Sweep of Integrated Micro Systemsxe2x80x9d by S. Tom Picraux and Paul J. McWhorter published December 1998 in IEEE Spectrum at pages 24 to 33.
One form of micro electromechanical devices in popular use are ink jet printing devices in which ink is ejected from an ink ejection nozzle chamber. Many forms of ink jet devices are known.
Many different techniques on ink jet printing and associated devices have been invented. For a survey of the field, reference is made to an article by J Moore, xe2x80x9cNon-Impact Printing: Introduction and Historical Perspectivexe2x80x9d, Output Hard Copy Devices, Editors R Dubeck and S Sherr, pages 207-220 (1988).
Recently, a new form of ink jet printing has been developed by the present applicant, which is referred to as Micro Electro Mechanical Inkjet (MEMJET) technology. In one form of the MEMJET technology, ink is ejected from an ink ejection nozzle chamber utilising an electro mechanical actuator connected to a paddle or plunger which moves towards the ejection nozzle of the chamber for ejection of drops of ink from the ejection nozzle chamber.
In previous designs of actuator a lower arm is deposited as a generally planar single layer, a sacrificial spacing layer is formed and then an upper arm is deposited as a generally planar layer.
A major portion of the cost of a manufactured wing semiconductor manufacturing techniques device depends on the number of separate layers required to be deposited during fabrication. Reducing the number of separate layers that need to be deposited reduces the cost of the device.
The efficiency of the thermal actuator is roughly inversely proportional to the mass of the actuator material. The actuator arms need to have a certain stiffness. If the stiffness of the arms can be maintained whilst decreasing mass, the efficiency of the actuator can be improved.
The present invention, in preferred forms, aims to address either or both of these issues.
In one aspect of the invention there is provided a thermal actuator which may be manufactured with material for upper and lower arm sets being deposited in a single step. This is achieved by having the arms arranged in a non-overlapping manner.
Accordingly, in one aspect the invention provides a thermal actuator for micro mechanical or micro electromechanical devices, the actuator including:
a supporting substrate,
an actuator extension portion,
at least one first arm attached at a first end thereof to the substrate and at a second end to the extension portion, the first arm being arranged, in use, to be conductively heatable,
at least one second arm attached at a first end to the supporting substrate and at a second end to the extension portion, the second arm being spaced apart from the first arm,
the first arm being arranged, in use, to undergo thermal expansion, thereby causing the actuator to apply a force to the extension portion, and
wherein, in plan view, said at least one first arm and said at least one second arm do not overlap.
In a second aspect of the invention there is provided a thermal actuator having at least one of the upper and lower arms including stiffening means.
Accordingly a second aspect of the invention provides a thermal actuator for micro mechanical or micro electromechanical devices, the actuator including:
a supporting substrate,
an actuator extension portion,
at least one first arm attached at a first end thereof to the substrate and at a second end to the extension portion, the first arm being arranged, in use, to be conductively heatable,
at least one second arm attached at a first end to the supporting substrate and at a second end to the extension portion, the second arm being spaced apart from the first arm,
the first arm being arranged, in use, to undergo thermal expansion, thereby causing the actuator to apply a force to the extension portion, and
wherein, in transverse cross-section, at least one of the first arm and the second arm are non-planar.
In transverse cross-section both the first and second arms may be non-planar.
The actuator preferably includes two first arms electrically interconnected at the second end and preferably includes three second arms. However, other combinations of upper and lower arms are within the scope of the invention.
The at least one first arm and at least one second arm may be spaced transversely relative to each other and preferably in plan view do not overlap.
The edges of the first and second arms may be located in a common plane.
Where edges of the first and second arms are located in a common plane the first arm preferably has a centre of inertia located to one side of the common plane and the second arm preferably has a centre of inertia located to the other side of the common plane.
In transverse cross-section the at least one first arm and/or the at least one second arm may include an edge portion extending away from the other arm. The edge portion may extend inwardly.
In transverse cross-section the at least one first arm or the at least one second arm, or both, may have a U, V, C or W profile.
In a third aspect of the invention there is provided a method of manufacturing a thermal actuator in which material for upper and lower arms is deposited in a single step.
Accordingly a third aspect of the invention provides a method of forming a thermal actuator including the steps of:
a) depositing a first layer of sacrificial material;
b) depositing at least a second layer of sacrificial material on a selected part or parts of the first layer;
c) depositing an actuator forming layer of material over the first and second layers of sacrificial layers; and
d) selectively removing portions of the actuator forming material to form at least one first arm deposited on the first layer and at least one second arm deposited on at least part of the second layer.
Step b) may include depositing one or more additional layers of sacrificial material on selected parts of the second layer.
The additional layer or layers may be deposited on only part of the second layer.
The at least one first arm so formed may be planar or non planar. In transverse cross-section the at least one first arm or the at least one second arm, or both, may have a U, V, C or W profile.
The layers may be deposited so that, when formed, the transverse edges of the at least one first arm lie in a first plane and the transverse edges of the at least one second arm lie in a second plane. The first and second planes may lie in a common plane.
In plan view the at least one first arm and the at least one second arm may overlap or may be spaced transversely to each other.
In plan view when there are at least two first or second arms the arms alternate.
In the preferred embodiments, an improved thermal bend actuator is formed through the depositing and etching of a number of layers with the actuator arms being in the form of a corrugated or non planar structure so as to provide for increased stiffness in the bending direction. The corrugated titanium nitride layer of the thermal bend actuator is defined utilising standard lithographic steps and in a number of embodiments is of such a nature that the masking edges occur on the same planar layer such that all edges will be simultaneously within the depth of field of a stepper process.
It is important that the arms of an actuator have sufficient stiffness to avoid buckling. For a given stiffness the efficiency of the thermal actuator is roughly inversely proportional to the mass of the actuator material. The utilization of a corrugated or non-planar structure provides for improved stiffness characteristics for a given thermal mass. The corrugations achieve a higher stiffness without increasing the thickness and thereby increasing the mass of the actuator. This provides an overall increase in efficiency. Generally, simulations indicate efficiency increases of around 50% are possible.
In the preferred embodiment, a thermal bend actuator is formed by the deposition of a single layer of titanium nitride so as to form spaced-apart actuator arms which do not coincide in a vertical plane. The advantage of utilising a single layer of out of plane actuator is that the number of layers to be deposited and patterned in the structure are substantially reduced.